Obesity is causally linked with numerous health problems. State-of-the-art behavioral interventions for obesity reliably facilitate clinically significant weight loss in the short-term, but weight regain (and a return to elevated disease risk is normative. Innovative interventions that improve long-term outcomes of lifestyle modification must be developed and tested. New evidence suggests that a high level of physical activity (PA) is a critical component of weight loss maintenance (and has numerous ancillary health benefits, independent of weight). Most adults who are prescribed high levels of PA (whether as part of a weight management program or other intervention) do not ultimately adhere to these prescriptions, suggesting that traditional behavioral skills and intervention strategies are not sufficiently matched to the challenges of this critical, long-term behavior change. Theoretical and empirical work strongly suggest that for obese adults to be successful at consistently engaging in high levels of PA they must employ specific psychological skills, including mindful decision-making, behavioral commitment in the face of challenges, an ability to tolerate physical and psychological discomfort, and an ability to achieve clarity about one's personal values. These skills are the focus of acceptance-based behavioral interventions for PA promotion. To test the effectiveness of this approach, this study will recruit 300 adults from the community and provide them with 6 months of group-based standard behavioral weight loss treatment (Phase I). In Phase II, participants will receive one of three interventions, to be delivered for an additional 12 months: 1) behavioral treatment, with the standard emphasis on maintaining changes in diet and PA (BT), 2) behavioral treatment, with a primary emphasis on using these skills to maintain PA (BT-PA), or 3) acceptance-based behavioral treatment, with a primary emphasis on using these skills to maintain PA (ABT- PA). The PA prescription will be uniform across conditions. All participants will be encouraged to gradually progress to and subsequently maintain an amount of PA that will result in 2000 kcal/wk of energy expenditure. Assessments will be conducted at baseline, 6 months (end of Phase I treatment), 18 months (end of Phase II treatment), 24 months (6-month follow-up), and 36 months (18-month follow-up). An intensive focus on PA after initial weight loss (i.e., BT-PA and ABT-PA) is hypothesized to result in a) better weight loss maintenance, and b) higher amounts of PA at post-treatment and follow-up compared to standard behavioral treatment (BT). An intensive focus on PA after initial weight loss is also hypothesized to result in a) better weight loss maintenance and b) higher amounts of PA at post-treatment and follow-up when acceptance-based behavioral skills are taught (i.e., ABT-PA), compared to when standard behavioral skills are taught (BT-PA). Theory- driven mediators and moderators of intervention effects also will be examined. Accomplishing these aims will advance research on health-related behavior change and has the potential to meaningfully impact the behavioral prevention and treatment of many obesity-related diseases and conditions.